This invention relates to a method of purifying exhaust gases from internal combustion engines, especially exhaust gases from motor vehicles, the purification being effected catalytically and serving to simultaneously remove hydrocarbons, carbon monoxide and nitrogen oxides from the gases when a nearly stoichiometric fuel-air ratio is being maintained. Such catalytic converters are well known in the art.
It is known that the gases which are exhausted from an internal combustion engine when the fuel-air ratio is nearly stoichiometric (.lambda. = 0.99 - 1.01) contain only a little carbon monoxide, hydrogen and unburned hydrocarbons, whereas their content of nitroen oxide is higher in comparison to a richer adjustment. On the other hand, an exhaust gas from the combustion of a nearly stoichiometric mixture also contains a quantity of oxygen which is roughly equivalent to the quantity of the components that have not been burned. Since it is technically possible to adjust fuel-air mixtures to stoichiometric proportions and to maintain these proportions during operation, an exhaust gas which is nearly free from carbon monoxide, hydrocarbons and nitrogen oxides will be obtained if the nitrogen oxides can be successfully catalytically reacted with some of the hydrogen or carbon monoxide present in the exhaust gas to form nitrogen, and the hydrocarbons and the remainder of the carbon monoxide present in the exhaust can likewise be reacted with the oxygen.
In principle the following reactions would be involved EQU CO + 0.5 O.sub.2 .fwdarw.CO.sub.2 ( 2) ##EQU1## EQU H.sub.2 + 0.5 O.sub.2 .fwdarw. H.sub.2 O (3) EQU CO + NO .fwdarw. CO.sub.2 + 0.5 N.sub.2 ( 4) EQU h.sub.2 + no .fwdarw. h.sub.2 o + 0.5 n.sub.2 ( 5)
if the stoichiometric proportion of the oxidizing to the reducing reactants in the exhaust gas is equated with unity, the following relationship will hold ##EQU2## In this equation (O.sub.2), (NO), (CO), (H.sub.2) and (C.sub.n H.sub.2n+2) are the molar concentrations of oxygen, nitric oxides, carbon monoxide, hydrogen and hydrocarbons, respectively, and n is an integer If the ratio L &lt; 1, the reducing components of the system are preponderant, whereas if the ratio L &gt; 1 there is an excess of oxidizing components. This ratio is hereinafter referred to as the L-value of the exhaust gas.
If the fuel-air ratio supplied to the vehicle engine is kept constant between about 0.99 and 1.01 by a control system of fuel injection, the exhaust gas will contain on the order of about 1% carbon monoxide, 0.4% hydrogen, 0.02% propane, and 0.1% nitric oxide, whereas the oxygen content will be from about 0.5 to 0.9%. If these limits for the oxygen concentration are introduced into equation (6), all other quantities remaining unchanged, then the values of L will be
L = 0.69, when O.sub.2 = 0.5% and PA1 L = 1.19, when O.sub.2 = 0.9%
and the stoichiometric ratio L = 1 will be attained when the oxygen content is 0.75%.
The effect of a catalyst for different L-values in the neighborhood of L= 1 (stoichiometric composition of the exhaust gas) is determined as follows:
The conversion percentages of nitrogen oxides (NO.sub.x), carbon monoxide and the hydrocarbons are plotted on a graph as functions of L. The variability range of L within which the conversion percentages of the three above-mentioned exhaust gas constituents exceed 90% (.DELTA.L) can be used as a criterion for the effectiveness of the catalyst and should be as wide as possible.
Catalysts which satisfy this criterion have already been described in German Offenlegungsschrift No. 2,304,831, and consist of a combination of platinum, ruthenium, rhodium and rhenium on a carrier which has been calcined at a temperature of not less than 800.degree. C.. These catalysts contain a relatively large proportion of noble metals and they are therefore expensive.
German Offenlegungsschrift No. 2,306,395 describes supported catalysts which comprise an intermetallic system consisting of at least three metals as the catalytically active component, the system including a platinum group metal and one base metal selected from aluminum, titanium, chromium, manganese, cobalt and nickel. Such catalysts also have a relatively high noble metal content.